Ubiquitin is a small protein that has important regulatory roles in a wide variety of cellular pathways. Ubiquitin chains linked through the lysine at position 11 (K11) have been identified as important regulators of cell division (Jin et al., 2008; Kirkpatrick et al., 2006) and have been implicated in signaling degradation of ubiquitin ligase anaphase-promoting complex (APC/C) substrates, an essential step in eukaryotic cell division (Jin et al., 2008; Williamson et al., 2009). The APC/C recruits two E2 enzymes, the ubiquitin chain-initiating UbCH10 and the chain elongating Ube2S, which assemble K11-linked chains with high specificity (Garnett et al., 2009; Williamson et al., 2009; Wu et al., 2010). Loss of this APC/C-specific E2 module leads to defects in mitotic progression (Williamson et al., 2009; Song and Rape, Molecular Cell in press). While these results suggest that K11-linked chains drive protein degradation by the proteasome during mitosis, characterization of ubiquitin chains assembled by the APC/C, UbCH10 and Ube2S has relied largely on in vitro experiments. Direct evidence of K11-linked polyubiquitin chains regulating protein degradation in cells has been lacking due to an absence of tools available to directly detect them.